Since cardiovascular diseases have accounted for top three of ten causes of death in Taiwan, public awareness on prevention of cardiovascular diseases is increasing. The people with the cardiovascular diseases include older age, hypertension, diabetes, hyperlipidemia, exposure to tobacco, obesity, and a family history of cardiovascular diseases, etc. The doctor diagnose patients by understanding the detail of patient disease history and checking with the equipment such as electrocardiogram (ECG), stress ECG, phonocardiogram, echocardiography, nuclear medical imaging study, cardiac computerized tomography (CT) scan, cardiac magnetic resonance imaging (MRI), etc. There are two major problems of heart valves: valvular stenosis, which is inadequate opening, and valvular regurgitation, which is backward leakage of blood through closed valves. These problems result in elevated pressure or increased volume in heart chambers, leading to deterioration of heart function.
Heart valvular system consists of mitral, tricuspid, aortic and pulmonary valves. Heart valves allow blood to circulate through them when they are open and prevent backward blood flow when they are closed. The mechanisms make unidirectional blood flow possible and preserve energy. Each valve can develop stenotic and regurgitant disorders, and some patients have valvular prolapse of mitral and tricuspide valves, which indicate elongated valves with leaflets prolapsed to the atrial chambers. The circulation of blood in cardiovascular system is as following: deoxygenated blood returns to the right atrium from peripheral venous system via the superior and inferior venae cavae (SVC and IVC). The right ventricle relaxes to allow blood to go into the right ventricle via the tricuspid valve and then contracts to pump blood in the pulmonary circulation via the pulmonary valve. After oxygen (O2) and carbon dioxide (CO2) exchange in the lungs, oxygenated blood returns to the left atrium. The left ventricle relaxes to allow blood to go into the left ventricle via the mitral valve and then contracts to pump blood in the systemic circulation via the aortic valve. During ventricular systole and diastole, forward flow is allowed and backward flow is prevented by functioning valvular opening and closing.
The most commonly available exam of cardiac diseases is ECG, which provides indirect evidences of valvular diseases. For example, aortic stenosis results in ventricular pressure overload and left ventricular hypertrophy. Thus an increased QRS amplitude, ST segment/T-wave abnormalities can be observed in ECG. However, the same findings can be found in other diseases. The evaluation of cardiac systolic and diastolic function requires further chest X-ray, echocardiography and nuclear medical tests. Cardiac murmurs generated by valve insufficiency can be heard using a stethoscope. There are some limitations: a physician being able to check a single auscultation site at a time and the narrow range of human hearing (20 Hz to 20 kHz). Some abnormal heart sounds, such as the third and the fourth heart sounds (S3 and S4) are relatively lower in frequency, which might be beyond the limits of human hearing and are missed in cases. Therefore, phonocardiography is used to record heart sound to check the opening and closing timing of heart valves. There is a time delay between heart valves closing and signal captured on body surface, leading to confusion in clinical evaluation. Chest X-ray reveals calcification of the valves, but it provides limited information of valvular heart diseases and is not a useful tool for continuous monitoring because of radiation. Echocardiography is a useful tool to evaluate cardiac contraction and valvular function. The commonly used echocardiographic modalities include M-mode, 2-D, 3-D, Doppler and contrast echocardiography images. Chamber sizes and valvular motion can be evaluated using M-mode echocardiography, and normal and abnormal blood flow can be detected using Doppler and contrast echocardiography. However, echocardiography machine is bulky, and professional personnel are required, making the exam very inconvenient and not suitable for continuous monitoring.
The conventional ways for checking heart valve defect include ECG, phonocardiogram, echocardiography, and nuclear medical tests. However, these techniques have their limitations while in use. Such as, the ECG can be used to estimate intervals of heart valve operation, but can't be used to check opening and closure of heart valves effectively. The phonocardiograms can be used for checking the opening and closing of heart valves, yet are unable to detect changes in blood flow of the heart. The echocardiography can be used to check the lumen diameter, the heart valve movement, the direction of blood flow, the velocity and turbulence of the blood flow in cardiac vessels, but poses problems in long term monitoring. Thus there is room for improvement and a need to provide a novel method for mechanocardiography that overcomes the shortcomings of conventional ways for checking heart valves. The method not only records heart valve operation and blood flow features for long-term monitoring but also improves convenience and accuracy in measurement.